1. Field of the Invention
The invention relates generally to enclosed switchgear and more particularly enclosed switchgear having removable drawout circuit breaker units.
2. Description of the Prior Art
Circuit interrupters having one or more drawout units mounted within an enclosure are operable between engaged and disengaged positions. Maintenance procedures are performed after rolling out the circuit breaker to disconnect it from the source of high voltage. The high voltages carried by the apparatus can produce injury to maintenance personnel and damage to the equipment if the circuit breaker is not positively located at both the connect or engaged and the disconnect or disengaged positions. For example if the high potential is reapplied to the switchgear when the circuit breaker has a poor connection at the engaged position, the resultant arcing would cause such intense heat as to cause damage to the switchgear apparatus, sometimes with explosive force, endangering personnel proximate the switchgear enclosure. Accordingly it is necessary that the drive means for moving the drawout unit between the engaged and disengaged position, have the ability to positively position the drawout unit at both the engaged and disengaged positions. This ability is necessary whether the drive means be a hand cranked mechanical mode or electrical motor operated optional mode for remote operation used for a hazardous or explosive environment. The most common method employed in the prior art for attaining this positive position was to operate the drive means whether by hand or electrical motor until the drive means "bottomed out" that is ceased to operate at each end of the drawout unit's free travel. This meant for the hand cranked mechanical drive means mode the operator would turn the crank until he met a large resistance. With the electrical motor operated drive mode the operator would energize the motor until the motor is stopped at the end of the free travel. The disadvantage of course in such a positioning scheme is that it would be difficult for the operator to determine whether the circuit breaker was indeed at the end of the free travel or if it had bound up on something lying in the travel path. Complicating the operator's determination is the fact that a rather large amount of resistance force is necessary to overcome the resilient fingers when the contact clusters initially encounter the fixed conductor stabs of the cabinet, after which the drawout unit must be driven further against lower resistance to be in the fully engaged position wherein maximum contact of the cluster/stab arrangement is attained. A friction or torque operated clutch placed in the drive means is an unsatisfactory solution to the above problems for the same reasons that the "bottomed out" approach is unsatisfactory, i.e. the variations in torque encountered along the travel path of the drawout unit and the possible binding of the travel mechanism render a friction or torque operated clutch an imprecise method of determining the engaged and disengaged positions. Accordingly it would be desirable to incorporate an isolatable position actuated drive clutch on the drive means for a circuit interrupter drawout unit so that it may be located in the low voltage area of the switchgear.